What is the difference between iter(obj) and using obj directly in a for-loop, in terms of what Python calls internally?

They are equivalent: a for-loop implicitly calls iter(obj) once to obtain an iterator, then calls next() on that iterator repeatedly until StopIteration. Calling iter() explicitly just lets you obtain and hold onto the iterator object yourself, for example to pass it to another function or advance it manually between loop iterations.

nums = [1, 2, 3]

# Implicit (for-loop does this internally)
for n in nums:
    print(n)

# Explicit equivalent
it = iter(nums)
while True:
    try:
        print(next(it))
    except StopIteration:
        break